How do i deal with my resignation after being taken advantage of ?

So im 25 m started working in this tabaco chain in my country for about 4 mounts now and immediately the first week i saw some red flags in my contract it said i work 40h but in reality i work 48h they immediately put on a ton of overtime in the first few weeks because they are desperate for workers because it pays close to minimum wage for long hours basically. So i worked basically and looked for a new at the same time till i recently found one and they asked me to start training asap and to talk to hr about maybe shortening the notice (in my country its 30 days ). Thing is I've worked basically freelance as a waiter in weddings parties etc so im new to working under contract so im not as well versed in workers law but i know i was taken advantage the overtime i work is not tracked in the book for attendance but we have to tell hr and they track it and basically give you the money in a envelope (its very little pay for the hours worked) and today i saw that i have hours missing from july and stuff like night shifts aren't payed and neather are the Saturdays. Today i quit but the hr lady cut me of repeatedly saying you are under contract and when i brought up the contract that i work more then on paper she said ou that doesn't matter and she said i need to finish my 30 days notice before i quit . So what do i do because i dont want thid new job to pass me and i dont whana work somewhere im taken advantage of but i also don't know what they could do to retaliate if i do just say i quit and stop coming to work I'd appreciate some advice thanks in advance .